Intermediate conifers - Department of Horticulture

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Intermediate conifers - Department of Horticulture
By Bert Cregg, Ph.D.
Michigan State University
Department of Horticulture and
Department of Forestry
October 2007
33
In the two most recent installments of Conifer Corner, I’ve been
discussing outstanding conifers in each of the four size categories
recognized by the American Conifer Society (ACS), beginning
with the smallest size category (Miniature conifers) and working
up to larger conifers. In this edition of Conifer Corner, I continue
this discussion by considering Intermediate conifers.
The ACS defines Intermediate conifers as those
that grow 6-12" per year and reach a height of 6'
to 15' at age 10. I refer to Intermediate conifers
in the title of this article as ‘the forgotten conifers’
because we often define Intermediates by what
they are not. They are not adorable little miniatures
or dwarfs that can fit into a container or rock
garden. Conversely, Intermediates won’t dominate
a landscape as a specimen, like a large conifer.
Nevertheless, intermediate conifers are an important
group of trees because they fulfill an important
function in landscape design and development.
Unlike their smaller cousins, miniature and dwarf
conifers, intermediate conifers are less likely to
get “lost in the shuffle” in a busy landscape
design. Unlike large conifers, intermediates are
less likely to interfere with power lines or other
overhead obstruction and are better suited to
plant closer to houses and allow greater flexibility
in site selection.
As with all of the trees that I’ve discussed in this
series on conifers in the ACS size classes, there is
considerable variation in growth depending on
site and cultural practices. This is especially true
with intermediates. Some intermediate conifers
growing on a good site could reach a size comparable
to large conifers. There is no single “official” trial
garden where conifer sizes are determined. Size
classes are based on observations by growers and
conifer enthusiasts and a given cultivar may be
listed as an intermediate in one reference and as a
large in another. In general, size classes referred
to in Conifer Corner articles are based on the
ACS Conifer Database.
Pinus cembra ‘Compacta
Glauca’ With a tight
compact form and straight
central leader it’s easy
to see why Chub Harper
gushes, “I never met a
cembra I didn’t like.”
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The Michigan Landscape™
Here are Intermediate conifers worth
considering for Michigan.
Swiss Stone pine, Pinus cembra. Conifer expert
Chub Harper has a passion for plants that is
matched by few, and few plants get him as
excited as Pinus cembra. Just mention Pinus
cembra and Chub goes all Will Rogers. “I’ve
never met a cembra I didn’t like,” Chub
enthuses. “This is one of those plants I just
don’t understand why we don’t use more.”
Indeed, it’s easy to understand Chub’s
swooning for Swiss Stone pine. This species
maintains a tight compact form with a straight
single leader, giving it a stately elegance
without pruning. Moreover, Pinus cembra has
beautiful blue green needles that often take a
silvery sheen. Cembra is fairly tolerant of a
range of sites and is variously listed as zone 3
or 4, so it is hardy in most of the Lower
Peninsula. Pinus cembra is one of those plants
that even the straight species is distinctive
enough to make an impression. In addition,
there are a number of cultivars on the market.
Note that some of the cultivars included here
fit into the Dwarf size class.
Pinus cembra ‘Silver Sheen’. A striking
cultivar of P. cembra with silvery blue
needles. Zone 5.
Pinus cembra ‘Chalet’ is a pyramidal to
upright form of Pinus cembra. There is a
terrific specimen in the Harper Collection.
Zone 4, though also reported to Zone 3.
(above) Intermediatesized conifers like Pinus
cembra ‘Chalet’ provide
character without taking
over a landscape.
(left) Pinus cembra
‘Silver Sheen’. Swiss
Stone pines are reliable
performers in Michigan
landscapes.
October 2007
35
Japanese White pine, Pinus parviflora is a solid
performer in Michigan. There are a variety of
cultivars of Japanese White pine, some of which
I’ve mentioned in earlier Conifer Corners.
Pinus parviflora ‘Bergman’. This is one of
the more striking forms of Japanese White
pine, with twisting blue-green needles.
Variously listed as a dwarf or an intermediate.
Pinus parviflora ‘Fukuzumi’. The form on
this plant can be variable, but its recurved
blue-green twisted needles give it consistent
appeal.
Golden Thread falsecypress, Chamaecyparis
pisifera ‘Filifera Aurea’. Yes, the Latin name
is a mouthful, but this plant is a show-stopper
like few others. ‘Filifera Aurea’ generate interest
from a distance due to their bright yellow
color and upright weeping form, as well as up
close due to their impossibly long thread-like
foliage.
(above) Striking gold. The
color and texture of
Chamaecyparis pisifera
‘Filifera Aurea’ make it a
show-stopper.
(right) Golden Thread
falsecypress (Chamaecyparis
pisifera ‘Filifera Aurea’)
welcome visitors to the
Harper Conifer Collection at
Hidden Lake Gardens.
Photo: Jack Wikle.
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The Michigan Landscape™
Abies concolor ‘Conica’ continues the theme of
color – this time striking blue. Abies concolor
‘Conica’ also continues the theme of underused
conifers. This tree’s stately upright form makes
it a prime choice as a specimen plant. Put one
of these in your yard and you’re guaranteed to
have all of the neighbors asking “Whatzzat?”.
Picea glauca ‘Pendula’. Technically this is listed
as an intermediate, but on better sites it may
push toward the large category. Also, this cultivar
has been around long enough that it’s possible
to find some decades-old larger specimens. The
tight form and drooping branches make it easy
to envision this plant in a snow-blanketed
mountainside.
underside. While that alone is enough to give
the tree tremendous ornamental appeal,
‘Silberlocke’, like most Korean firs, also
produces prodigious amounts of colorful cones.
Although cones on firs make Christmas tree
growers cringe, in this case it adds to the plant’s
landscape appeal.
Picea glauca ‘Pendula’ makes
an elegant specimen tree.
Abies koreana ‘Silberlocke’ gets double marks
for showmanship. The needles on this cultivar
of Abies koreana are highly recurved, that is,
they are turned upward to reveal their silvery
Abies concolor ‘Conica’ is a striking upright form of Concolor
fir that is well adapted in Michigan.
October 2007
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Ornamental conifer size classes recognized by the
American Conifer Society
Category
Growth per year1
Miniature
Approx. size at
10 years2
< 1"
< 1'
Dwarf
1" to 6"
1' to 6'
Intermediate
6" to 12"
>6' to 15'
>12"
>15'
Large
Size may vary due to cultural, climatic and geographical region
Refers to growth in any direction
Source: American Conifer Society (www.conifersociety.org)
1
2
Corkbark fir, Abies lasiocarpa var. arizonica.
An argument for the lumpers and splitters. Some
references list this as a variety or even sub-species
of Subalpine fir (A. lasiocarpa) while others
(for example, the Gymnosperm database www.
conifers.org) list Corkbark fir as its own species
(A. bifolia). In the nursery trade, var. arizonica
holds sway. Another high elevation conifer
form the mountain Southwest, Corkbark fir is a
striking tree that can often match Picea pungens
for blue color.
Larix kaempferi ‘Pendula’. Deciduous conifers
always make a unique contribution to the
landscape and this Weeping larch is no
exception. This plant commands interest
anytime during the growing season, but
especially in the early spring when the new
bright green needles are just beginning to
flush. According to the ACS, this tree is widely
mislabeled as Larix decidua ‘Pendula’.
Need a break from ‘blue spruce burnout’? Abies lasiocarpa
var. arizonica ‘Compacta’ has outstanding form and color.
38
The Michigan Landscape™
Walnut Glen Colorado blue spruce, Picea
pungens ‘Walnut Glen’ is an interesting twist
on the old standby, Colorado blue spruce.
This P. pungens cultivar has a golden cast to its
needles, which often make it look like the sun
is shining on it even when it’s in the shade.
Reportedly adapted to a fairly wide range of
site conditions though the variegated yellow
needles may suffer scorch under stress.
(clockwise from top left) Bright spot. The yellow highlights of Picea pungens ‘Walnut Glen’
can brighten up even a dreary fall day in Michigan.
Larix kaempferi provides a dramatic contrast in form and color.
Deciduous conifers such as Larix decidua ‘Julian’s Weeper’ provide year-round interest in
the landscape.
October 2007
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Thuja occidentalis ‘Degroot’s Spire’. In
general, arborvitae are the kinds of plants that
don’t usually get people too excited. However,
this upright columnar form makes a great
accent and can add a formal appearance as a
border or can be grouped for effect.
(above) Columnar conifers
such as Thuja occidentalis
‘Degroot’s Spire’ are
especially dramatic when
grouped for effect.
(below) No need to
spray… Conspicuous resin
dots are characteristic of
Pinus aristata needles, but
are sometimes mistaken
for pine scale.
40
Bristlecone pine, Pinus aristata. We can’t really
recommend bristlecone pine as an outstanding
grower in Michigan, but it’s a fascinating tree
and it will grow on suitable sites here in Michigan.
In their native habitat, Bristlecone pines are
among the oldest living things on earth. The
Rocky mountain form (P. aristata) can live to
nearly 3,000 years old and specimens of the
Great Basin form (P. longaeva) have been
found that are over 4,800 years old. Although
we don’t expect a Bristlecone pine to live
thousands of years in Michigan (and, in any
case, we won’t be around to see it), this makes
an interesting specimen in the right spot. Look
for a site with good drainage and relatively
good air flow. Like many trees adapted to the
arid west, these pines don’t like wet feet or
high humidity.
Thuja occidentalis ‘Hetz Wintergreen’. Another
upright form of arborvitae. ‘Hetz Wintergreen’
is noteworthy for a couple of reasons. It tends
to maintain its green color throughout the
winter when many other conifers turn offcolor and it tends to maintain a strong central
leader, whereas many columnar arbs can
develop multiple leaders or bend over under
snow loads. Excellent for a year-round
screening hedge and windbreak.
The Michigan Landscape™
Weeping Serbian spruce, Picea omorika
‘Pendula Bruns’. It’s always hard to go wrong
with a Serbian spruce. ‘Pendula Bruns’ is a little
slower growing and has a little tighter form than
‘Pendula’ and ‘Berliners Weeper’ — more on
those two in the next Conifer Corner.
Black Hills spruce, Picea glauca ‘Densata’.
Depending on the reference, Black Hills spruce
is listed as either a true botanical variety (var.
densata) or simply as a cultivar. Regardless of
the taxonomy, this is a versatile tree that we’ll
likely see more of in the future. As the Latin
name ‘Densata’ implies, Black Hills spruce has a
tight, dense growth habit and maintains a nice
pyramidal form. Its growth rate is slower than
the straight species or blue spruce so it’s less
likely to get out of hand and provides more
flexibility in site selection. With its uniform
compact growth, Black Hills spruce also shows
promise as a table top Christmas tree.
Dr. Bert Cregg is an Associate Professor in the
Departments of Horticulture and Forestry at MSU.
Korean pine (shown
here as Pinus koraiensis
‘Rowe Arboretum’)
combines adaptability,
good growth, and good
form into one package.
Chub’s Choices
Chub Harper lists his ‘top five’ in each of
the ACS conifer size classes.
INTERMEDIATE CONIFERS —
Growth per year: six to twelve inches.
Size at age ten years: Six to fifteen feet.
1. Abies concolor ‘Conica’, Upright
Concolor fir
2. Chamaecyparis pisifera ‘Filifera
Aurea’, Yellow Thread Leaf falsecypress
3. Larix kaempferi ‘Pendula’, Weeping
Japanese larch
4. Pinus cembra, Swiss Stone pine
5. Pinus parviflora ‘Bergman’, Bergman’s
Japanese White pine
Barking up the right tree. The
passing years add even more
character to conifers like this
Pinus densiflora.
October 2007
41

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